Steelmaking additive composition

ABSTRACT

Steelmaking additive compositions and process of use, said compositions providing flux solubilization, desulfurization and/or removal of inclusions. The compositions comprise from about 25 to 50 wt % lime and about 75 to 50 wt % of a compound selected from the group consisting of bauxite and mixtures of calcium aluminate and bauxite; alternatively, the compositions comprise from about 40 to 70 wt % CaO, 20 to 50 wt % Al 2  O 3  from bauxite or mixtures of calcium aluminate and bauxite and 0 to 30 wt % MgO. The process consists of the addition, in the course of steelmaking utilizing electric furnace, basic oxygen furnace, AOD furnace or open hearth furnace means, of the steelmaking additive compositions of the invention to either the furnace, ladle and/or tundish.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.431,540, filed Sept. 30, 1982.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to steelmaking, particularly to additivecompositions for use during the steelmaking process.

2. Description of the Prior Art

The basic steelmaking process comprises a well-known series of generalsteps in the transformation of cast or pig iron into steel. Variousfurnaces for use in steelmaking are available, including the electricarc furnace, the basic oxygen furnace (top blown, bottom blown, top andbottom blown), the AOD furnace, and the open hearth furnace. Whilechoice of the specific furnace to be used and specific procedure to befollowed varies based on criteria known to those skilled in thesteelmaking art, depending upon the composition, purity and end-use ofthe steel desired, certain difficulties in steel production exist acrossthese broad procedures.

Primary among the problems faced in steelmaking is the removal of sulfurand inclusions, such as undesirable oxides, silicates and aluminates,from the steel during production. Removal of sulfur is of particularimportance and difficulty, as too high a sulfur content deleteriouslyaffects the performance of the resulting steel. Flexibility as to themanner and timing of effecting desulfurization and inclusion removal isalso important, yet not readily controllable. Ideally, the steelmakerwishes to be afforded the option to perform desulfurization throughtreatment of the initial charge in the furnace, before tapping; orthrough treatment of the steel in the ladle; or through treatment in thetundish.

Of similar importance is the ability to effect the solubilizing in thesteel charge of lime and/or other flux materials. Ease in solubilizingfluxing materials in the furnace affects the efficiency, control andspeed of the initial melting and heat operations, and impacts on theoverall composition, quality and purity of the steel product.

Many solubilizing, desulfurization and inclusion-removing steelmakingadditives are known to the art, principal among them being the use ofspar, CaF₂. While spar affords the flexibility as to the manner andtiming of effecting desulfurization and inclusion removal, its usepresents serious drawbacks.

When used to effect desulfurization in the furnace, spar attacks thefurnace's refractory lining, particularly eroding the refractory at theslag line. This drastically reduces the number of heats which the liningcan withstand before a costly and time-consuming relining of the vesselis necessary. When used to effect desulfurization in the ladle, sparagain attacks the refractory lining, necessitating frequent, costlyrelines and unacceptable downtime for the ladle. Alloy recovery duringinjection or stirring is also reduced as a result of spar use. A furtherproblem is the excessive heat loss that results from the use of spar inthe ladle, as it has poor insulating properties, requiring that the heatbe tapped at a high temperature and thereby resulting in a shortening offurnace and ladle refractory lining life. An additional shortcoming isthe ineffectiveness of spar to remove inclusions in the ladle,especially during stirring.

When used in the tundish, spar usage results in even greaterdifficulties. It fails to provide any cover material, resulting inexcessive heat loss and a poor temperature profile due to its inferiorinsulation properties. Use of spar in the tundish also fails to provideeffective desulfurization, nor does it remove and retain inclusions.Refractory attack is again a serious problem. Finally, varying degreesof undesired reoxidation and recarburization (if carbon is present) canoccur.

It is apparent that there existed in the steelmaking art a serious needfor an additive capable of solubilizing fluxing materials, desulfurizingthe steelmaking charge, and removing inclusions, such as undesiredoxides, aluminates, silicates and sulfides, without presenting theserious shortcomings resulting from use of the known additives, yetproviding the steelmaker with the flexibility to effect solubilizationof fluxes, desulfurization and/or inclusion removal either in thefurnace, ladle or tundish.

SUMMARY OF THE INVENTION

The aforenoted shortcomings of the known materials are alleviated by useof the compositions and method of the present invention, which providean additive composition effective for the solubilizing of fluxingmaterials, desulfurization, and removal of undesired inclusions from thesteelmaking charge during steelmaking operations through introduction inthe furnace, ladle and/or tundish.

The novel compositions of the invention comprise from about 25 to 50 wt%lime and 75 to 50 wt% of a compound selected from the group consistingof bauxite and mixtures of calcium aluminate and bauxite. Dolomitic lime(approximately 58 wt% CaO, 42 wt% MgO) may be utilized, as may standardlime (95 wt% CaO); either lime may be utilized in calcined or hydratedform, calcined being preferred. The calcium aluminate may be eithermono, di or tri calcium aluminate, with dicalcium aluminate (CaAl₂ O₄ orCa(AlO₂)₂) being preferred. The bauxite may be either straight or fusedbauxite, or mixtures thereof, with fused (or calcined) bauxite beingpreferred.

Raw material for the novel compositions of the invention includestandard compounds of acceptable quality and purity to the steelmakingindustry, as well as waste by-product material resulting from theproduction of calcium metal, e.g. slag material from the meltingprocess. Other possible sources include slags of the lime-aluminavariety generated in the production of stainless or alloy steel or othermetals.

The process of the invention comprises the addition of the noveladditive compositions of the invention during the steelmaking process toeffect flux solubilization, desulfurization and/or the removal ofinclusions. The compositions of the invention may be added to thefurnace with the initial charge, or at any time during the heat prior totapping. Alternately, or in addition to the furnace, the compositions ofthe invention may be added to the ladle or in the tundish.

The amount of the composition of the invention utilized is thatnecessary to effect the desired degree of flux solubilization,desulfurization and/or removal of inclusions. When used in the furnace,the amount of the compositions of the invention utilized will be 25 to50% of the heretofore known amount of spar utilized by those skilled inthe art. When used in the ladle, the amount of the compositions of theinvention utilized will be from about 5 to about 20 pounds per ton ofsteel, the specific amount used depending on the degree ofdesulfurization required, the surface area of the steel liquid in theladle, and the thickness of the blanket (or cover) of the compositiondesired to serve an additional insulating function to avoid heat lossand maintain a desired temperature profile in the molten steel. Whenused in the tundish, the amount of the compositions of the inventionutilized will be from about 1 to about 10 pounds per ton of steel, thespecific amount used depending on the degree of desulfurization and/orinclusion removal desired, the necessity to minimize reoxidation of thesteel, the surface area of the steel liquid in the tundish, and thethickness of the blanket (or cover) of the composition desired to servean additional insulating function to avoid heat loss and maintain adesired temperature profile in the molten steel.

The compositions of the invention afford the desired flexibility of useto the steelmaker without the concomitant shortcomings of the knownprior art additives, particularly spar. When used in the furnace, thecompositions of the invention effectively put lime and/or other fluxingmaterials into solution rapidly and efficiently, provide desulfurizationof the charge, and cleanse the charge of the known deleteriousinclusions, including undesirable aluminates, silicates and oxides. Whenused in the ladle, the compositions of the invention providedesulfurization of the charge, cleanse the charge of the knowndeleterious inclusions, and provide an insulating blanket (or cover) tominimize heat loss and maintain a desired temperature profile. Finally,when used in the tundish, the compositions of the invention prevent orminimize reoxidation of the steel, provide desulfurization of thecharge, cleanse the charge of the known deleterious inclusions, andprovide an insulating blanket (or cover) to minimize heat loss andmaintain a desired temperature profile.

Accordingly, it is an object of this invention to provide novelsteelmaking compositions and process which provide flexibility ofaddition during the steelmaking cycle of the composition to effectsolubilization of flux materials, desulfurization and/or removal ofundesired inclusions.

It is a further object of this invention to provide novel steelmakingcompositions and process for adding said compositions to the steelmakingfurnace wherein use of said compositions avoids attack on the furnacerefractory lining, markedly improves the lining life while avoidingerosion at the slag line and reaction with the refractory, rapidlysolubilizes lime and/or other fluxes into the steelmaking charge, andeffects desulfurization and/or removal of inclusions.

It is another object of this invention to provide novel steelmakingcompositions and process for adding said compositions to ladles used inthe course of steelmaking operations wherein use of said compositionsavoids attack on the ladle refractory lining, markedly improves thelining life while avoiding erosion at the slag line and reaction withthe refractory, improves alloy recovery and efficiency, especiallyduring desulfurization, provides for control and minimization of heatloss and temperature profile by providing an insulating blanket (orcover), which affords tapping of the heat at a lower temperature (whichin turn improves furnace and ladle refractory lifetimes), and effectsdesulfurization and/or removal of inclusions during taping and stirring.

It is yet another object of this invention to provide novel steelmakingcompositions and process for adding said compositions to the tundishused in the course of steelmaking operations wherein use of saidcompositions markedly reduces or eliminates reoxidation orrecarburization of molten steel, avoids attack on the tundish refractorylining, markedly improving the lining life, provides for control andminimization of heat loss, and control of temperature profile, byproviding an insulating blanket (or cover), effects or enhancesdesulfurization, and provides excellent removal of inclusions andcleansing of the molten steel.

Other objects and advantages of this invention will become apparent uponreading the following detailed description and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred proportion of the compounds comprised within thecompositions of the invention is about 33 to 67wt% lime and 67 to 33 wt%of a compound selected from the group consisting of bauxite and mixturesof calcium aluminate and bauxite. A particularly preferred compositionof the invention consists essentially of about 50 wt% lime and 50 wt% ofa compound selected from the group consisting of bauxite and mixtures ofcalcium aluminate and bauxite. An optimal composition of the inventionconsists essentially of about 25 wt% lime and 75 wt% of a compoundselected from the group consisting of of calcium aluminate and bauxiteand mixtures thereof.

The preferred compositions of the invention may also utilize dolomiticlime, which is made up of approximately 58 wt% CaO and 42 wt% MgO. Thepreferred compositions of the invention may utilize mono, di or tricalcium aluminate; dicalcium aluminate (CaAl₂ O₄ or Ca(AlO₂)₂), however,is particularly preferred for use in these compositions. Bauxite is alsoparticularly preferred for use in these compositions. The mixtures ofcalcium aluminate and bauxite may be used at any proportion, one to theother, through from 50 to 90 wt% calcium aluminate and 10 to 50 wt%bauxite is preferred.

The optimal compositions of the invention utilize only bauxite as analumina source or consist of about 50wt% lime, about 35wt% calciumaluminate and about 15wt% calcined bauxite.

Specific preferred compositions of the invention are the following, theAl₂ O₃ content being provided by bauxite or mixtures of calciumaluminate and bauxite:

    ______________________________________                                        CaO                 50 to 70 wt %                                             Al.sub.2 O.sub.3    20 to 50 wt %                                             MgO                 OPTIONAL                                                  SiO.sub.2           <3 wt %,                                                  particularly                                                                  CaO                 65%                                                       Al.sub.2 O.sub.3    33%                                                       SiO.sub.2           <3%                                                       MgO                 OPTIONAL;                                                 CaO                 55-65 wt %                                                Al.sub.2 O.sub.3    30-40 wt %                                                MgO                 OPTIONAL                                                  SiO.sub.2           <3 wt %,                                                  particularly                                                                  CaO                 60 wt %                                                   Al.sub.2 O.sub.3    35 wt %                                                   SiO.sub.2           <3 wt %                                                   MgO                 OPTIONAL;                                                 CaO                 50-60 wt %                                                Al.sub.2 O.sub.3    35-45 wt %                                                MgO                 OPTIONAL                                                  SiO.sub.2           <3 wt %,                                                  particularly                                                                  CaO                 57%                                                       Al.sub.2 O.sub.3    39%                                                       SiO.sub.2           <3%                                                       MgO                 OPTIONAL.                                                 ______________________________________                                    

Specific particularly preferred compositions of the invention, utilizingdolomitic lime, are the following:

    ______________________________________                                        CaO                 45-55 wt %                                                Al.sub.2 O.sub.3    20-30 wt %                                                MgO                 15-25 wt %                                                SiO.sub.2           <3 wt %                                                   particularly                                                                  CaO                 51 wt %                                                   Al.sub.2 O.sub.3    26 wt %                                                   MgO                 22 wt %                                                   SiO.sub.2           <3 wt %                                                   CaO                 50-60 wt %                                                Al.sub.2 O.sub.3    30-40 wt %                                                MgO                 10-20 wt %                                                SiO.sub.2           <3 wt %,                                                  particularly                                                                  CaO                 52 wt %                                                   Al.sub.2 O.sub.3    35 wt %                                                   MgO                 15 wt %                                                   SiO.sub.2           <3 wt %,                                                  CaO                 45-55 wt %                                                Al.sub.2 O.sub.3    35-45 wt %                                                MgO                 5-15 wt %                                                 SiO.sub.2           <3 wt %,                                                  particularly                                                                  CaO                 48 wt %                                                   Al.sub.2 O.sub.3    39 wt %                                                   MgO                 11 wt %                                                   SiO.sub.2           <3 wt %,                                                  ______________________________________                                    

An optimal composition of the invention is as follows:

    ______________________________________                                        CaO                 40-60 wt %                                                Al.sub.2 O.sub.3    35-55 wt %                                                SiO.sub.2           <3 wt %                                                   MgO                 <1 wt %                                                   Fe.sub.2 O.sub.3    <1 wt %                                                   N.sub.2             <.2 wt %                                                  CaF.sub.2           OPTIONAL                                                  C                   trace                                                     P                   trace                                                     S                   trace,                                                    particularly                                                                  CaO                 50-60 wt %                                                Al.sub.2 O.sub.3    35-45 wt %                                                SiO.sub.2           <3 wt %                                                   MgO                 <1 wt %                                                   Fe.sub.2 O.sub.3    <1 wt %                                                   N.sub.2             <.2 wt %                                                  C                   trace                                                     P                   trace                                                     S                   trace.                                                    ______________________________________                                    

The preferred compositions of the invention may be used in combinationwith spar, CaF₂, substituting in part for that amount of spar previouslycalled for to effect flux solubilization, desulfurization and/or removalof inclusions. Use of the preferred compositions of the inventiontogether with spar ameliorates the deleterious effects of sparutilization, while affording the advantageous properties of thecompositions of the invention.

The preferred processes of the invention comprise the addition of thepreferred compositions during the process of steelmaking utilizingelectric furnace, basic oxygen furnace, AOD furnace or open hearthfurnace means. The preferred composition, in the necessary quantity, maybe added directly to the furnace utilized with the initial charge, or atany time prior to the tapping of the heat. The preferred compositionsbecome incorporated in the slag layer and provide solubility for limeand/or other fluxing materials, while effecting desulfurization and theremoval of undesired inclusions. The slag layer may be removed, or leftin the furnace, utilizing techniques known to those skilled in thesteelmaking arts, which results in the entrapment of the deleteriousmaterials in the slag.

Alternatively, the preferred compositions of the invention may be addedto the steel layer after tapping of the heat and its transfer to theladle. The composition is added to the ladle so as to form a blanket (orcover) layer over the molten steel. The composition draws the undesiredsulfur and/or inclusions out of the steel and into the blanket layer,while simultaneously serving to insulate the molten steel and preservedesired heat and maintain the necessary temperature profile in themetal. The overlayer formed by the composition, together with any otherslag or extraneous material, may be removed, or left in the ladle,utilizing techniques known to those skilled in the steelmaking arts,which results in the entrapment of the deleterious materials in theblanket or overlayer.

The preferred compositions of the invention may also be added to thetundish. As with addition to the ladle, tundish addition results in theformation of a blanket (or cover) layer over the molten steel. Thecomposition draws the undesired inclusions out of the steel and into theblanket layer, while simultaneously preventing reoxidation andrecarburization and serving to insulate the molten steel, preservingdesired heat and maintaining the necessary temperature profile in themetal. The presence of the preferred composition in the tundish alsoenhances desulfurization of the molten steel. The overlayer formed bythe composition, together with any other slag or extraneous material,may be removed, or left in the tundish, utilizing techniques known tothose skilled in the steelmaking arts. Again, use of the preferredcompositions results in the entrapment of the deleterious materials inthe blanket or overlayer.

The preferred compositions of the invention may be used in combinationwith other known steelmaking additives and additive compositions,including fluxing materials, desulfurizing compounds, degassingcompounds, and other such materials.

While particular embodiments of the invention, and the best modecontemplated by the inventor for carrying out the invention, have beenshown, it will be understood, of course, that the invention is notlimited thereto since modifications may be made by those skilled in theart, particularly in light of the foregoing teachings. It is, therefore,contemplated by the appended claims to cover any such modifications asincorporate those features which constitute the essential features ofthese improvements within the true spirit and scope of the invention.

I claim:
 1. A steelmaking additive composition consisting essentially offrom about 25 to 50 wt% lime and about 75 to 50 wt% of a compoundselected from the group consisting of bauxite and mixtures of calciumaluminate and bauxite.
 2. The composition of claim 1 consistingessentially of about 50 wt% lime and about 50 wt% of compositionselected from the group consisting of bauxite and mixtures of calciumaluminate and bauxite.
 3. The composition of claims 1 and 2 wherein saidlime is dolomitic lime.
 4. The composition of claim 3 wherein said limeis calcined.
 5. The composition of claims 1, 2, 3 or 4 consistingessentially of about 50 wt % lime, 35 wt % calcium aluminate and 15 wt %bauxite.
 6. A steelmaking additive composition consisting essentially offrom about 40 to 70 wt% CaO, 20 to 50 wt% Al₂ O₃ from bauxite ormixtures of calcium aluminate and bauxite and 0 to 30 wt% MgO.
 7. Thecomposition of claim 6 consisting essentially of about 65 to 70 wt% CaO,25 to 30 wt% Al₂ O₃ from bauxite or mixtures of calcium aluminate andbauxite, and 0 to 30 wt% MgO.
 8. In the process of steelmaking utilizingelectric furnace, basic oxygen furnance, AOD furnace or open hearthfurnace means, the improvement consisting of the addition of asteelmaking additive composition consisting essentially of from about 25to 50 wt% lime and about 75 to 50 wt% of a compound selected from thegroup consisting of bauxite and mixtures of calcium aluminate andbauxite.
 9. The process of claim 8 wherein said composition consistsessentially of about 50 wt% lime and about 50 wt% of a compound selectedfrom the group consisting of bauxite and mixtures of calcium aluminateand bauxite.
 10. The process of claims 8 or 9 wherein said lime in saidcomposition is dolomitic lime.
 11. The process of claim 10 wherein saidlime in said composition is calcined.
 12. The process of claims 8, 9, 10or 11 wherein said composition consists essentially of about 50 wt%lime, 35 wt% calcium aluminate and 15 wt% bauxite.
 13. In the process ofsteelmaking utilizing electric furnace, basic oxygen furnace, AODfurnace or open hearth furnace means, the improvement consisting of theaddition to the molten steel of a steelmaking additive compositionconsisting essentially of from about 40 to 70 wt% CaO, 20 to 50 wt% Al₂O₃ from bauxite or mixtures of calcium aluminate and bauxite and 0 to 30wt% MgO.
 14. The process of claim 13 wherein said composition consistsessentially of about 65 to 70 wt% CaO, 25 to 30 wt% Al₂ O₃ from abuxiteor mixtures of calcium aluminate and bauxite, and 0 to 30 wt% MgO.